Sunday, January 5, 2014

"In a canyon..." It's been decades since I've been on a formal trip to this area, and I didn't have a print copy of a guide, so our stops were somewhat random, dictated by pull-outs as they became visible. I did find a nice guide to the geology of this area earlier today, aimed at a less technical audience. Stop 3, to which the link will take you, can be thought of as roughly the boundary between the lower crust magma chamber in a spreading ridge (back-arc, not mid-ocean), and the overlying middle crust, dominated by sheeted dikes.

Above is a very simple schematic showing the idealized rock sequence in the Josephine Ophiolite. In reality, it's cut by several major faults, so sequences are missing in some areas, and repeated in others. Additionally the boundaries between the major rock environments are much more gradational than this communicates, although the guide itself does a very good job of explaining that. Looking carefully at the lead photo, I think we're seeing a few individual dikes cutting through the gabbro of the upper portion of the magma chamber. From the view here, though, this did not look like a sensible spot to inspect the rocks more closely.

Photo unmodified. May 8, 2013. FlashEarth location. (Uncertain- the geometry of the scene and placement in the geological sequence look right, as well as the presence of a pull-out, but there were no landmarks for me to nail down where I was.)